The present invention relates to cellular communications systems and, more particularly, to a method and apparatus for estimating the time of arrival of a spread spectrum signal in a wireless communication system.
Location determination and tracking of a remote or mobile unit within a wireless communication system, such as a code division multiple access (CDMA) system, has many applications, including emergency, e.g., 911, services for wireless telephone subscribers, location sensitive billing, fleet management, and so forth. It is well known that the distance of a mobile unit from a base station can be determined by analyzing the propagation delay, or time of flight, of a signal transmitted from the mobile unit to the base station, or vice versa. Likewise, the location of a mobile unit within a wireless communication system can be determined using a trilateration method, such as time of arrival (TOA) and time difference of arrival (TDOA) techniques. According to such a methods, distances between the mobile unit and multiple, e.g., three, base stations are calculated based on a measurement of time delay of a signal traveling between the mobile unit and each base station. When determining the location of a mobile unit is desired, a signal transmitted from the mobile unit to multiple base stations. The time of arrival (TOA) is estimated at each base station, and TOA or TDOA locating techniques are used to analyze the propagation delay differences and calculate a distance from each base station to the mobile unit. Since the location of the each base station is precisely known, the location of the mobile unit can be determined. Also, the propagation delay for a single base station can be used to determine or track the distance of a mobile unit from the base station. TOA information for a single base station can also be combined with angle of arrival information to determine location.
The effects of a radio channel upon the correlation result of a CDMA signal transmitted thereon can result in a delay spread of about 10-15 xcexcs. Such effects are well known and include phase error, fading, multi-path propagation, motion effect (Doppler shift, multi-path dynamics), and so forth. Any location estimation scheme generally involves the estimation of the TOA of the leading ray. The mobile unit uses the leading finger""s position to estimate the distance to the base station, whereas the base station uses the round trip delay information to locate the mobile unit. The leading finger uses its delay locked loop (DLL) element to track the peak energy signal.
The drawback of DLL peak tracking for TOA estimation, however, is that the time at which the peak energy is located does not always accurately represent the TOA of the signal. This is because the TOA is defined as the arrival time of the leading edge of the signal. On average, the use of peak energy location for arrival time estimation results in an error of about 300-400 ns with high signal strength in IS-95 based systems, thus causing error in location estimation.
Prior art techniques for enhancing accuracy of estimation of the arrival time of the leading ray include using separate circuitry to search in front of the leading finger position to find the prompt ray for angle of arrival estimation, or, using of a bank of correlators before the peak energy location and watching the energy readings out of the correlator array. However, both techniques demand extensive additional circuitry, making them more costly to implement. Also, since such techniques require accumulation of data over time, it is very difficult to use such techniques for tracking the position of a mobile unit in real time.
A need therefore exists for an improved tracking apparatus and method providing enhanced TOA estimation for locating mobile units in a wireless communication system.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention.